System for image acquisition

ABSTRACT

A system for image acquisition for acquiring images includes at least two optical devices for acquiring images, each optical device including a respective linear camera suitable for acquiring images from at least one object passing on a transporting plane, at least one lighting device associated with each of the at least two cameras and suitable for lighting the said object in regions of the object from which the images may be acquired, the lighting devices light the object with light pulses, the light pulses being synchronized so that light generated by the at least one lighting device associated with a camera does not interfere with the acquisition of another of the at least two cameras, the cameras acquiring the images only when the respective at least one lighting device is active.

The invention relates to a system for image acquisition by means ofoptical camera type devices for image acquisition, particularly fixedoptical devices.

In the present disclosure and in the subsequent claims the expression“optical device for image acquisition” is intended for a device capableof acquiring images of an object and particularly optical informationassociated with an object placed on a supporting plane, e.g. objectidentifying data, such as an optical code associated with the object.

The expression “optical information” is intended for any graphicalrepresentation constituting a coded or un-coded information. Aparticular example of optical information consists of linear orbi-dimensional optical codes, wherein the information is coded by meansof suitable combinations of elements of predetermined shape, for examplesquares, rectangles or hexagons, of dark colour (normally black)separated by clear elements (spaces, normally white), such as the barcodes, the stacked codes and the bi-dimensional codes in general, thecolour codes, etc. The expression “optical information” furthercomprises, more generally, also other graphical shapes, includingprinted or hand-written characters (letters, numbers, etc.) andparticular shapes (so-called “patterns”), such as stamps, logos,signatures, fingerprints, etc. The expression “optical information” alsocomprises graphical representations detectable not only in the range ofthe visible light, but also in the entire range of wave lengthscomprised between infrared and ultraviolet.

It is known from the prior art to use, in systems for image acquisition,cameras comprising mono-dimensional (linear) array of photo-sensors,particularly of CCD or C-MOS type, for acquiring the images of parcels,or objects in general, travelling on a conveyor belt, or other handlingand transporting systems, and reading via said cameras the opticalinformation printed or affixed thereon, or extracting from said imagesvarious information about the objects, such as the volume or sizes.

The expression “fixed optical device for image acquisition” is intendedfor an optical device for image acquisition that is used without humanmanipulation (so-called “unattended scanner”). The object detectiontypically comprises reading an optical code and also, possibly,measuring a distance and/or a volume or other dimensional properties ofthe moved objects.

The systems for image acquisition known from the prior art typicallycomprise at least a telecamera (or simply camera) and a lamp or solidstate based lighting system. In most cases, then, one or more reflectingmirrors are present. These components may be accommodated in a commoncontainer or in separated containers.

The camera has the function of collecting the image from which theinformation for identifying an object has to be extracted. Such imagemay be the image of the object as a whole or an optical code—as definedabove—contained therein. The image acquisition occurs by means of asuitable optical system and a dedicated opto-electronics andelectronics, wherein an optical sensor exists consisting of a CCD or aC-MOS of linear type comprising an array of photo-sensitive elements(also called pixels).

The image is acquired by storing subsequent scans, each of whichrepresents a thin “line” of the whole image. The movement of thesupporting plane, or the object, at the fixed reading station, enablessubsequent lines of the image to be acquired and, then, the completeimage to be acquired.

The lighting system enables the acquisition region to be lighted withthe appropriate light levels and lighting angles.

The deflecting mirror, or the deflecting mirrors, enables theinstallation of the device for image acquisition to be optimised fromthe point of view of the space occupied with respect to the devicetransporting the objects and enables the field of view of the camera(defined in the following), and possibly also the beam of light emittedby the lighting system, to be oriented towards the desired region.

As already said, the camera acquires the image of the object row by rowand transmits said image to a decoder that reconstructs the imageacquired by the camera by assembling all the rows, and then processessaid image in order to extract (decode) the information of the opticalcodes and/or other information or send said image or make said imageavailable for a further processing apparatus. The decoding algorithmperforms a bi-dimensional analysis of the acquired images whereby a codehaving any orientation can be properly read. For this reason, the camerasystems having linear sensors are considered omnidirectional acquiringand reading systems.

The image acquisition is controlled by a microprocessor, that,typically, is accommodated into the camera, but can be also external andconnected with said camera. The microprocessor receives information fromexternal sensors, such as object height sensors, object presencesensors, speed sensors, and uses this information to regulate as good aspossible the operating parameters of the camera, such as sensitivity,position of the autofocus system, scanning speed, etc.

The high resolution of the used cameras and the high speed at which theobjects normally move, typically between 0.8 and 3 m/s for applicationsconcerning recognising, tracking and sorting objects, require opticalconfigurations with very short exposure times, therefore very opendiaphragms and, consequently, a low depth of field. In order to be ableto acquire images and read optical information in a wide range ofcamera-object distances, as typical for industrial applications (forexample for identifying and sorting parcels), the camera is usuallyprovided with an autofocus system wherein the receiving optical system(or a part thereof), or the sensor, moves to modify the focalisationparameters of the camera and enabling reading of optical information onobjects of different shapes and dimensions. Usually, the autofocussystem of the camera “follows” the shape of the objects on the base ofthe information about the height provided by the height or distancesensors, such as barriers of photocells.

The expression “depth of field” is used herein to indicate the range ofcamera-object distances, in a neighbourhood of the distance of perfectfocalisation set each time by the autofocus system, wherein the objectis sufficiently focused in order to enable the optical information to beread.

As mentioned above, the camera needs some essential information forproperly setting its operational parameters, such that the opticalinformation associated with the moving objects is acquired.

Particularly, the camera has to know the object speed. Usually, when,for example, the transporting device is a conveyor belt or a trayconveyor, the speed sensor is an optical encoder associated with theconveyor belt that generates a square wave whose frequency isproportional to the speed of the belt. Actually, the encoder is a sensorof the advancing belt, from which the speed of the tape and consequentlythe speed of the objects is obtained by derivation.

Furthermore, for a correct and effective operation of the autofocussystem, the camera has to know the height of the objects or, when saidcamera is a camera designed for reading codes on a side face of theobjects, said camera has to know the lateral position of the objects.Height and distance sensors are then provided, such as photocellbarriers and laser sensors that measure the time of flight of theemitted laser beam, said sensors being placed upstream the camera(s).

The camera must especially know when acquisition of the sequence ofrows, or lines, constituting the image (so-called “frame”) has to bestarted, and how long the acquisition has to last. In systems with aplurality of cameras it is furthermore necessary that every object hasan unambiguous identification for all the cameras. For this reason, allthe cameras of the system share the same source of “frame trigger” forstarting the acquisition of the sequence of rows. This source istypically a presence sensor (for example a photocell) that detects thepresence of an object on a horizontal line perpendicular to thedirection of the conveyor belt and generates the signal of “frametrigger”. Alternatively, the height sensor may be provided as the deviceof “frame trigger”. The signal of “frame trigger” is generated when themeasured height exceeds a certain predefined threshold.

The start and the end of the “frame” acquisition are determined from astart/stop signal generated by the “frame trigger” device. However,acquisition does not start as soon as the “frame trigger” device detectsan object, but starts with a delay predetermined for every camera of thesystem, said delay depending on the distance between the “frame trigger”device and the view line of the camera on the plane of the belt, theview angle of the camera, the speed of the objects and the measuredheight thereof.

All the sensors disclosed above can be physically connected to thecamera(s) or to a control device that processes the information and“distributes” said information to the camera(s).

The control device controls all the sensors and can also control thelighting devices.

The information provided by the sensors is distributed to the camerasand every camera, on the base of said information and the positioning ofthe camera itself, adapts its own acquisition parameters. Particularlyeach camera, on the base of the information on the speed of the objectsor the advancing signal of the conveyor belt received from the controlcircuit, regulates its own acquisition frequency (or the scanningfrequency, i.e. the number of lines acquired per second), so that thespatial distance between two subsequent lines remains constant. For thispurpose, in the known systems, all the cameras independently generate adistinct starting signal of the acquisition of each single line of image(so-called “line trigger”).

When the acquisition has stated, the camera adapts its own acquisitionparameters to the actual condition. Particularly: the acquisitionperiod, and therefore the acquisition frequency, depends on the speed ofthe objects and can also depend on the height thereof; the position offocus, where autofocus system is present, has to be adapted as much aspossible to the shape of the objects; and the sensitivity of the cameradepends on the distance or height of the objects (high objects areusually more lighted) and the speed of said objects.

These parameters are typically modified continuously during acquisition.

Every camera is typically connected with a respective decoder thatprocesses the sequence of the acquired lines and decodes the opticalcode or transmits the image to a distinct processing and recognisingapparatus. The decoders can communicate each other using for example aTCP/IP protocol on Ethernet. One of the decoders functions as master andcollects the data from the different decoders and sends said data to thehost. Alternatively, a separated device can be provided (for example aPC) that collects the data from the decoders and transfers said data tothe host for subsequent processing.

When face of the objects on which the code is placed is not known orwhen one or more codes are present on a plurality of faces of theobjects, a multi-side system, or a system with multiple readingstations, with a plurality of cameras, has to be provided.

In a multi-side system, for example with five cameras, a camera acquiresthe image of the upper surface of the object. Four side cameras acquirethe images of the side surfaces. Each camera is coupled with a differentlighting device (or a plurality of lighting devices) accommodated in acontainer separated from the camera. Alternatively, each camera may havethe lighting device or the lighting devices integrated in the samecontainer. For reasons of space the cameras and the devices of laterallighting, frequently do not light/read directly, but via deflectingmirror.

A “Bottom” camera can also be provided that is to be positioned underthe conveyor belt, so that the view plane, properly oriented by means ofone or more deflecting mirrors if the case may be, passes through thespace separating two sections of belt and, therefore, intercepts thelower surface of the objects when said objects pass from a section ofthe belt to the other. Therefore, this camera should be designed forreading codes arranged on the lower faces of the objects. The multiplestation systems known from the prior art show the drawback that thereading stations take a large amount of space in the direction of lengthof the conveyor belt, since the regions lighted by different lightingdevices have not to overlap each other.

In fact, the lighting devices in said systems operate continuously, i.e.each lighting device associated with the respective camera turns on assoon as it receives an appropriate signal from the respective camera orthe respective decoder, or the respective control device of the system,said signal indicating that an object entered the reading region of thesystem, and remains turned on during the whole reading time of therespective camera.

Should the regions lighted by different lighting devices overlap eachother, any image acquired by a camera in the overlapping region would beover-lighted and/or affected by reflections deriving from the lightingdevice of the neighbouring camera, with the risk of compromising readingof the image.

This problem can not be compensated by using the sensitivity control ofthe camera, since the brightness variation on the edges of theoverlapping region is very rapid and unpredictable and the reflectionsinside this region depend on the orientation angle of the object on theconveyor belt and the material with which the surface of the object ismade.

In the known systems therefore, overlapping of the lighting planes andthe view planes of the cameras is avoided. That involves a remarkablelengthening of the reading stations along the transporting direction ofthe objects and is felt as a great disadvantage, since usually, intypical applications such as sorting and tracking applications ofparcels and objects, the reading stations should be confined in spacesas more limited as possible.

The present invention intends to obviate the disadvantages indicatedabove.

According to the present invention, a system for image acquisition isprovided comprising at least two optical devices for acquiring images,each optical device comprising a respective linear camera suitable foracquiring images from at least an object travelling on a transportingplane, at least one lighting device associated with each of said atleast two cameras and suitable for lighting said object in regions ofsaid object from which said images are suitable for being acquired,characterised in that said lighting devices light said object with lightpulses, said light pulses being synchronised so that light generated bysaid at least one lighting device associated with a camera does notinterfere with the acquisition of another of said at least two cameras,said cameras acquiring said images only when the respective at least onelighting device is active.

Owing to the invention the length requirement of the system for imageacquisition can be significantly reduced, since it possible to make surethat beams of light generated by different lighting devices do notinterfere with each other, even if the view planes of the respectivecameras cross each other, since, owing to the lighting with light pulsesmutually synchronised, the beams of light reflected or scattered by theobject in the fields of view of different cameras can not interfere witheach other. That enables the reading stations to be positionedremarkably closer to each other.

The invention will be described here below, with reference to theattached drawing, wherein:

FIGS. 1 and 2 are sketched views showing a system for image acquisitionaccording to the prior art, using a single camera;

FIG. 3 is a sketched top view of a system for image acquisitionaccording to the prior art, using a plurality of cameras;

FIG. 4 is a diagram showing the operation of the system for imageacquisition of FIG. 3;

FIG. 5 is a diagram showing the view planes of the cameras of the systemof FIGS. 3 and 4;

FIG. 5 a is a perspective sketched view showing an arrangement ofcameras to be avoided in the systems of the prior art;

FIG. 6 is a plan view of a system for image acquisition according to theinvention;

FIG. 7 is a diagram showing the view planes of the cameras of the systemfor image acquisition according to the invention;

FIG. 8 is a diagram showing the operation of the system for imageacquisition according to the invention;

FIG. 9 is a diagram showing a first example of the timing of the camerasin the system for image acquisition according to the invention;

FIG. 10 is a diagram showing a second example of the timing of thecameras in the system for image acquisition according to the invention.

In FIGS. 1 and 2 a system for image acquisition according to the priorart is diagrammatically shown, which uses a single camera 1, placedabove a transporting plane 2, for example a conveyor belt, on whichobjects 3 travel. The camera 1 is arranged for reading identifyingcodes, for example a bar code, printed or applied on the upper face 7 ofthe object 3. The camera 1 is associated with two lighting devices 4,for example LED or solid state or lamp based lighting devices ingeneral, lighting the region (generally a plane) inside which the camera1 has to perform the reading. The camera 1 can read the codes placed onthe upper surface 7 of the object 3 both directly and via a mirror 5,that is used when the camera can not be arranged, or properly placed fora direct reading.

The camera 1, owing to its autofocus system, substantially focuses oneline at a time of the reading region on the upper surface 7 of theobject 3. The line of perfect focalisation is called view line andrepresents the projection of the sensor of the camera 1 through theoptical receiving system of the camera, at the distance of perfectfocalisation. The set of the view lines (or reading lines) at thevarious distances allowed by the autofocus system constitutes thereading field, also called field of view, of the camera 1. The readingfield lies on a plane called view plane V (FIG. 2). The angle α betweenthe view plane V and a plane P perpendicular to the plane of theconveyor belt 2 is called reading angle, or view angle.

For a proper and effective operation of the autofocus system, the camera1 needs to know the height of the objects 3. Along the conveyor belt 2,upstream the camera 1, in the advancing direction of the objects 3, aheight sensor 6 is therefore provided, for example a barrier ofphotocells, or a laser sensor, detecting the height of the incomingobject 3.

Furthermore, a speed sensor 9 is associated with the conveyor belt 2(FIG. 4), for example an encoder, for detecting the speed at which theobjects 3 move, in order to regulate the sensitivity and the acquisitionspeed of the camera 1 on the basis of said speed.

The start and the end of the acquisition of the sequence of rows of theimage (“frame”) by the camera 1 are set by means of start/end signalsgenerated by a “frame trigger” device, represented by presence sensors10 (FIG. 4) or by height sensors 6 placed along the conveyor belt 2.However the acquisition does not start as soon as the “frame trigger”device detects an object, but starts with a predetermined delay,depending on the distance D between the “frame trigger” device (in FIGS.1 and 2 represented by height sensor 6) and the intersection of the viewplane V of the camera 1 with the plane of the conveyor belt 2, on theview angle of the camera, on the speed of the objects and on themeasured height thereof.

In FIG. 3 a system for image acquisition according to the prior art isshown using five cameras, so as to be able to read optical codes printedor applied on the upper surface or any one of the four side faces of anobject travelling on a conveyor belt 2, in the direction indicated bythe arrow F.

The system comprises:

-   -   an upper camera la associated with respective lighting devices 4        a and a respective mirror 5 a, through which the upper camera 1        a reads information associated with the upper surface of a        passing object, along a view plane Va (in the Figure the whole        field of view of the camera la is represented);    -   a left front camera 1 b (with respect to the direction of the        arrow F), associated with a respective lighting device 4 b and a        respective mirror 5 b, through which the camera reads        information associated with the front face and the left side        face of a passing object, along a view plane Vb;    -   a left rear camera 1 c, associated with a respective lighting        device 4 c and a respective mirror 5 c, through which the camera        reads information associated with the rear face and the left        side face of a passing object, along a view plane Vc;    -   a right front camera 1 d, associated with a respective lighting        device 4 d and a respective mirror 5 d, through which the camera        reads information associated with the front face and right side        face of a passing object, along a view plane Vd;    -   a right rear camera 1 e, associated with a respective lighting        device 4 e and a respective mirror 5 e, through which the camera        reads information associated with the rear face and the right        side face of a passing object, along a view plane Ve.

In FIG. 4 the operation of the system for image acquisition shown inFIG. 3 is shown.

The cameras 1 a-1 e are actuated by a control device 8 that is connectedwith the height sensor 6, that detects the height of the objectsarriving in the reading region of the cameras 1 a-1 e, the speed sensor9, that detects the speed at which the incoming objects move, thepresence sensor 10, that detects the arrival of an object near thereading region of the cameras 1 a-1 e and is used for generating the“frame trigger” signal, and the distance sensors 11 that are used fordetecting the distance of the object from the edges of the conveyor beltand for determining the orientation of the object on the conveyor belt.

The control device 8, on the basis of the sensor readings, distributessuch information to the cameras 1 a-1 e and controls the switching on ofthe lighting devices 4 a-4 d. On the base of the information receivedfrom the control device 8, each camera regulates the focalisation andthe image acquisition speed, establishes the time at which the imageacquisition has to be started and regulates its own sensitivity.

The images acquired by each camera, in the form of a series of imagelines, are sent to a respective decoder 12 associated with the camera,which reconstructs the image acquired by the camera by assembling allthe rows of the image and processes said image for extracting theinformation of the optical codes and/or other information. Dataprocessed by each decoder are then sent, for example through a hub 13,to a data processing system 14, for example a personal computer, forstoring and possible other processing.

FIG. 5 illustrates a diagram showing the view plane Va of the camera 1 aand the view planes Vb-Ve of the cameras 1 b-1 e, in order to highlightthat said view planes must not cross each other so that the regionslighted by lighting devices 4 a-4 e are prevented from crossing togethercausing over-lighting in the crossing regions and/or reflections thatcould disturb the image acquisition by the cameras 1 a-1 e.

For sake of explanation, reference is made to FIG. 5 a illustrating acamera arrangement that must be avoided in prior art systems having aplurality of cameras. Only two cameras of a prior art system with aplurality of cameras are shown in the Figure, for example the right rearcamera Td, with the respective lighting devices Id and the left rearcamera Ts with the respective lighting devices Is. The cameras Td andTs, as shown in FIG. 5 a, are arranged so that the respective viewplanes Vd and Vs cross on the rear face of the object 3 so as to formangles of 90°. The light incident on the rear face of the object 3, andoriginating from the lighting devices Is of the left rear camera Ts canbe reflected in the field of view of the right rear camera Td so as tocompromise the reading thereof.

In FIG. 6 a system for image acquisition according to the invention isshown, that uses five cameras, similarly to the prior art system shownin FIG. 3, to read optical codes printed or applied on the upper surfaceor any of the four side faces of an object travelling on a conveyor belt102, in the direction indicated by the arrow F1.

The system according to the invention comprises:

-   -   an upper camera 101 a associated with respective lighting        devices 104 a and a respective mirror 105 a, through which the        upper camera 101 a reads optical information associated with the        upper surface of a passing object, along a view plane LVa;    -   a left front camera 101 b, associated with a respective lighting        device 104 b and a respective mirror 105 b, through which the        camera reads optical information associated with the front face        and the left side face of a passing object, along a view plane        LVb;    -   a left rear camera 101 c, associated with a respective lighting        device 104 c and a respective mirror 105 c, through which the        camera reads optical information associated with the rear face        and the left side face of a passing object, along a view plane        LVc;    -   a right front camera 101 d, associated with a respective        lighting device 104 d and a respective mirror 105 d, through        which the camera reads optical information associated with the        front face and the right side face of a passing object, along a        view plane LVd;    -   a right rear camera 101 e, associated with a respective lighting        device 104 e and a respective mirror 105 e, through which the        camera reads optical information associated with the rear face        and the right side face of a passing object, along a view plane        LVe.

As it is possible to see from the Figure, the cameras and the mirrorsare arranged so that the view planes of each camera cross the viewplanes of the neighbouring cameras. For example the view planes LVb andLVd, of the cameras 101 b and 101 d, respectively, and the view planesLvb and LVc of the cameras 101 b and 101 c, respectively, cross eachother. That enables the volume of the system according to the inventionto be considerably reduced in the advancing direction of the conveyorbelt 102, with respect to prior art systems like that shown in FIG. 3,which is particularly advantageous when the system has to be installedin environments with a reduced available space.

FIG. 7 shows a diagram that illustrates more in detail the crossings ofthe view planes of the cameras.

That is made possible because, unlike the systems of the prior art, thelighting devices 104 a-104 e in the system according to the invention donot emit light continuously, but by pulses, which enables each lightingdevice to light the view line of the respective camera, while the viewlines of the other cameras, or a part of the other cameras are notlighted. Each camera 101 a-101 e acquires images only when therespective lighting device 104 a-104 e is active. That prevents a regionof view of each camera from being over-lighted and removes the risk thatthe image acquisition of a camera can be disturbed by light reflectionsfrom the view regions of the other cameras.

For example, if the cameras 104 a-104 e are divided into two groups ofcameras, respectively A and B, wherein the view planes of the cameras ofeach group do not interfere with each other, and a scanning time t isestablished, the system can be programmed so that the cameras of anygroup acquire a row during one half t/2 of the scanning time then turnoff their own lighting devices and leave the cameras of the other groupto acquire a row of image during the other half t/2 of the time.

The upper camera 101 a, the view line of which does not interfere withthe view lines of the other cameras, can be assigned indifferently tothe group A or B.

For example, the group A can comprise the left front camera 101 b andthe right rear camera 101 e, the view planes of which LVb and Lve do notinterfere, being substantially parallel, while the group B can comprisethe left rear camera 101 c and the right front camera 101 d, the viewplanes of which LVc and LVd are substantially parallel.

The upper camera 101 a, as said, can belong, indifferently, to the groupA, or the group B, since its view line LVa does not intersect the viewlines of the other cameras. Should a further upper camera be present,the view plane of which intersected the view plane of the camera 101 a,the two upper cameras would clearly belong to different groups.

All the cameras 101 a-101 e acquire images at the same scanningfrequency.

The acquisition instant is computed on the basis of a reference signalS, so-called “line trigger”, generated by a so-called master controldevice that is the same for all the cameras, one of the cameras beingable to take the role of master device, for example the upper camera 101a, as shown in FIG. 8. The “line trigger” signal, the same for thecameras of the two groups, can be a square wave signal having a periodT.

The master camera receives the information from the height sensors 6,the presence sensors 10, the speed sensors 9 and the distance sensors11, by means of a junction box 108 and generates the “line trigger” and“distributes” said “line trigger” to the other cameras. In the Figure,the decoders and the connections with the data processing system are notshown: these components may be arranged and connected as in the knownsystems of FIG. 3.

The diagram of FIG. 9 shows a first timing example of the imageacquisition by the cameras 101 a-101 e.

At every “line trigger” pulse, each camera 101 a-101 e turns on its ownlighting device for a time interval corresponding to its own acquisitiontime and, during this period, opens its own electronic shutter andacquires an image line. When the electronic shutter is closed the imageline is downloaded from the CCD and processed in known manner.

The cameras of the group A acquire the image by starting the acquisitionon the rising front of the square wave signal of the “line trigger”, theacquisition lasting the time T_(acq). The cameras of the group B startthe acquisition with a delay T_(r) with respect to the cameras of thegroup A. The delay T_(r) is greater or equal to the acquisition timeT_(acq) of the cameras of the group A, so that the cameras of the groupB start to acquire images only after the cameras of the group A havestopped acquiring and their lighting devices have been turned off. Thecameras of the group B acquire the image for a time T_(acq1), that maybe greater than T_(acq), or also different therefrom, provided thatT_(r)+T_(acq1)<T. If also this condition is respected, it is assuredthat the cameras belonging to two different groups will never turn onsimultaneously their own lighting devices and never acquire imagessimultaneously.

According to a preferred embodiment, the delay Tr is selected equal tothe half of the minimum acquisition time T_(min)[s] for the specificapplication, defined as MinResolution[mm]/V_(max)[mm/s], whereinMinResolution is the minimum resolution of the camera, i.e. thedimension of the pixel in mm at the maximum distance of the object withrespect to the camera, and V_(max) is the maximum speed of the objects.In this embodiment, T_(acq) and T_(acq1) are both shorter than or equalto T_(min)/2 and T is longer than or equal to T_(min). It is alsopossible to imagine different, but equivalent timings.

For example, with reference to FIG. 10, if the “line trigger” is asquare wave having a period T, the acquisition of the group B of camerascould start on the descending front of the signal, i.e. with a delayequal to T/2 with respect to the start of acquisition by the cameras ofthe group A, being T/2>T_(acq), where T_(acq) is the acquisition periodof the cameras of the group A. In this event, the conditionT/2+T_(acq1)<=T has to be respected, which is equivalent to sayT_(acq1)<=T/2, so that the cameras belonging to two different groupsnever turn on simultaneously their own lighting devices and neveracquire simultaneously images.

For both the timing examples disclosed above, it is to be noticed that,when the speed of the objects increases, the period of the “linetrigger” generated by the master is reduced up to a minimum limitT_(min), that is reached when the speed of the objects is the maximumspeed allowed.

The acquisition times of the cameras belonging to a same group can alsobe different from each other. In this case, T_(acq) and T_(acq1) of thetiming examples disclosed above are to be considered as the maximumacquisition times of the cameras of the respective groups.

A further advantage connected to the fact that the lighting devices 104a-104 e emit light by pulse instead continuously, lies in that saidlighting devices remain turned on only when the respective camera isreally acquiring images, which reduces the energy required for theoperation of the lighting devices, with respect to the systems of theprior art, in which the lighting devices remain continuously turned on,as long as there are objects passing on the conveyor belt.

Furthermore, if the conveyor belt stops, in the system according to theinvention the lighting devices may be turned off, for being then turnedon again when the conveyor belt restarts, with a further energy saving.

In the practical embodiment, the materials, the dimensions and theoperative details can be different from those indicated, but technicallyequivalent thereto, without thereby departing from the juridical domainof the present invention.

For example, a lower camera may be provided, placed below the conveyorbelt 102, capable of reading optical information carried on the lowerface of the passing objects, when these latter pass on the gapseparating from each other contiguous conveyor belts. When this lowercamera has a view plane that does not conflict with the view planes ofthe other cameras, as the upper camera 101 a, said lower camera can beassigned indifferently to the group A or to the group B of cameras, fortiming the image acquisition.

Furthermore, in image acquisition systems comprising a greater number ofcameras and lighting devices, more than two groups may be provided, thecameras being assigned to the different groups on the base of theorientation of the respective fields of view and lighting planes.

Furthermore, the cameras may comprise one or more corresponding lightingdevices inside the respective containers so as to constitute morecomplete optical apparatuses for image acquisition, with a clearadvantage in terms of compactness of the system. Finally, the reflectingmirrors may be completely absent, or be associated only to a few camerasof the system.

Finally, even if the use of the present invention has been disclosedparticularly in systems for acquiring and reading optical information,the present invention can be advantageously used in systems foracquiring images in general, irrespective of the capability of acquiringand reading optical information.

Such systems may be, for example, camera systems, and associatedlighting devices, for acquiring images of objects moving by means of atransporting device, said images being to be sent to video-codingdevices, or for determining dimensional and shape properties of theobjects, or for being used with controlling and watching systems.

1. A system for image acquisition comprising at least two opticaldevices for acquiring images, each optical device comprising arespective linear camera for acquiring images from at least one objectpassing on a transporting plane, at least one lighting device associatedwith each camera for lighting said object in regions of said object fromwhich said images may be acquired, wherein said lighting devices lightsaid object with light pulses, said light pulses being synchronised sothat light generated by said at least one lighting device associatedwith a camera does not interfere with the acquisition of another of saidcameras, said cameras acquiring said images only when the respective atleast one lighting device is active.
 2. The system according to claim 1,wherein said cameras comprise a first camera positioned for acquiringsaid images from at least one upper surface of said object, a secondcamera positioned for acquiring said images from a front face and from afirst side face of said object, a third camera positioned for acquiringsaid images from said first side face and from a rear face of saidobject, a fourth camera positioned for acquiring said images from saidfront face and from a second side face of said object and a fifth camerapositioned for acquiring said images from said second side face and fromsaid rear face of said object.
 3. The system according to claim 2,further comprising a sixth camera positioned for acquiring said imagesfrom a lower face of said object.
 4. The system according to claim 1,wherein said cameras are arranged so that the fields of view of at leasttwo of said cameras intersect each other.
 5. The system according toclaim 1, wherein the start and the duration of the light pulses emittedby each of said lighting devices are set by a control device operativelyassociated with said cameras.
 6. The system according to claim 5,wherein one of said cameras carries out the functions of said controldevice.
 7. The system according to claim 5, wherein said control devicegenerates a periodic reference signal on the basis of which the startand the duration of the light pulses of said lighting devices aresynchronised.
 8. The system according to claim 7, wherein said periodicreference signal is a square wave.
 9. The system according to claim 8,wherein the start of a first light pulse emitted by said at least onelighting device associated with one of said cameras corresponds with thestart of the rising front of said square wave.
 10. The system accordingto claim 9, wherein the start of a second light pulse emitted by said atleast one lighting device associated with another of said at least twocameras is delayed for a delay time with respect to the start of saidfirst light pulse.
 11. The system according to claim 10, wherein saiddelay time is equal to half of the period of said reference signal. 12.The system according to claim 10, wherein said first light pulse has aduration equal to the duration of said second light pulse.
 13. Thesystem according to claim 10, wherein said first light pulse has aduration different from the duration of said second light pulse.
 14. Thesystem according to claim 10, wherein the sum of said delay time and theduration of said second light pulse is shorter than the period of saidperiodic reference signal.
 15. The system according to claim 1, furthercomprising a height sensor for detecting the height of objects passingon said transporting device.
 16. The system according to claim 1,further comprising a presence sensor for detecting the presence of saidobjects in a predetermined position along said transporting device. 17.The system according to claim 1, further comprising a speed sensor fordetecting the advancing speed of said objects on said transportingdevice.
 18. The system according to claim 1, further comprising adistance sensor for detecting the distance of said object from tworeference planes parallel to the advancing direction of said object. 19.The system according to claim 5, further comprising a junction box forcollecting signals generated by two or more of a height sensor, apresence sensor, a speed sensor and a distance sensor and for sendingsaid signals to said control device.
 20. The system according to claim1, further comprising a plurality of decoders wherein a respectivedecoder is operatively associated with each camera.
 21. The systemaccording to claim 20, wherein each of said decoders is operativelyassociated with a data processing means.
 22. The system according toclaim 1, wherein a reflector is associated with at least one camera,said reflector directing light reflected by said object towards saidcamera.
 23. The system according to claim 1, wherein said cameras areoperable for acquiring optical information associated with said at leastone object.
 24. The system according to claim 1, wherein said at leasttwo optical devices for acquiring images are fixed optical devices.